In the past, extrusion molding compositions used asbestos and water-soluble cellulose ether as molding additives for enabling effective molding of parts having surface smoothness and strength. The late legal regulations ban asbestos. Often pulp fibers are used as the replacement of asbestos. Since pulp fibers are less dispersible, it is a common practice to use water-soluble cellulose ether in more amounts than before for the purpose of improving dispersion and strength or facilitating extrusion molding.
While water-soluble cellulose ether serves as a binder, it has the drawback that at a temperature in excess of 40° C., it undergoes thermal gelation, loses the binder function, and causes cracks and meandering stream during molding. Namely, compositions are difficult to mold at high temperature.
Under the circumstances, Patent Document 1 discloses an asbestos-free extrusion molding composition comprising cement and a water-soluble cellulose ether. Thermal gelation is prevented by restricting the degree of methoxy substitution to a range of 1.0 to 2.5 and the molar substitution of hydroxyalkyl to a range of 0.25 to 2.5. This is effective particularly when the hydroxyalkyl group is hydroxyethyl.
Patent Document 2 describes hydroxypropyl methyl cellulose in which a ratio of the degree of methoxy substitution to the molar substitution of hydroxypropyl is from 4.5 to 6.0, and specifically extrusion molding using hydroxypropyl methyl cellulose having a degree of methoxy substitution of up to 1.5.
Since water-soluble cellulose ether also serves as a setting retarder for cement, a problem arises when it is added to a cement-based extrusion molding composition. A long cure time is necessary when the molded part is allowed to stand and cure until the cement-based composition is set.
Solutions to this problem are known. Intending to shorten the cure time, Patent Document 3 proposes a cement-based extrusion molding composition to which a mixture of water-soluble cellulose ether and a setting accelerator such as calcium chloride is added.
Patent Document 4 discloses combined use of water-soluble cellulose ether and polyisoprene sulfonic acid compound. Satisfactory plasticity is available even when the amount of water-soluble cellulose ether added is reduced as compared with the water-soluble cellulose ether used alone.
Although Patent Documents 1 and 2 indicate improvements in thermal gelation temperature, they refer nowhere to the setting retardation effect of water-soluble cellulose ether. Although Patent Document 3 is successful in shortening the cure time through the combined use of a setting accelerator such as calcium chloride, the combined use undesirably reduces the thermal gelation temperature of water-soluble cellulose ether.
When added to extrusion molding compositions, water-soluble cellulose ether must meet both the functions of plasticity and water retention. If the amount of water-soluble cellulose ether added is reduced as in Patent Document 4, extrusion molding becomes difficult.